High global demand, national security and climate change issues related to conventional fossil fuels have contributed to a need for more environmentally benign fuels. As such fuel-cells such as proton exchange membrane fuel cell (PEMFC) and solid oxide fuel cell (SOFC) have, been explored. However, while both are believed to be energy efficient and clean energy generating systems, they each require a low-sulfur fuel. One such low-sulfur fuel is hydrogen. Unfortunately, however, hydrogen distribution is currently lacking since transfer of hydrogen through pipelines in heavily populated areas may prove dangerous.
Another alternative fuel source for some fuel cells is pipeline natural gas. However, a potential problem with unprocessed pipeline natural gas is that it often may have up to 4-5% hydrogen sulfide (H2S) along with other sulfur compounds. Processing of natural gas to remove sulfur may be carried out by amine absorption. However, the amine absorption processing may leave residual H2S as a contaminant at concentrations of, for example, 5-10 ppm. In addition to H2S, unprocessed pipeline natural gas also often contains other organic sulfur species that may be added as odorants. Common odorants include mercaptans (e.g., ethyl, isopropyl, and tertiary butyl), thiols (e.g., tetrahydrothiophene), and sulfides (e.g., dimethyl, diethyl). Thus, the concentration of sulfur in odorized natural gas can be in the range of a few ppm to as high as 30-60 ppm. And unfortunately, common noble metal catalysts for e.g. Nickel, Platinum, and Palladium which are commonly used for reforming may be poisoned by these sulfur compounds when the concentration levels exceed even 1 ppm.
When the concentration of sulfur compounds is on the order of unprocessed pipeline natural gas described above and the scale of operation is relatively small such as in fuel cells, energy intensive processes like amine absorption are not commercially attractive means of removing sulfur compounds. Other methods such as packed bed sorption using, for example, activated carbon is unattractive since activated carbon becomes pyrophoric when exposed to natural gas while using various metal oxides in packed bed sorption processes may require a high temperature. Accordingly, new methods and materials for removing low level contaminants from a gas stream are needed.
Methods for removing high levels of impurities have been described in the literature. For example, WO 2009/3171 describes a complex, multi-step process for making a hollow sorbent fiber which has a lumen within the fiber. The hollow fiber is then lined in a post-treatment step by pumping a dilute solution of polyvinylidene latex solution (40% or less by volume to avoid blocking the fiber bore) through the fibers followed by pumping the bore with humid nitrogen to avoid pinholes. The lumened fibers are then “capped” at both the ends near the potting seals. The lumened, capped hollow fibers may be packaged for removing CO2 from flue gas in a relatively high feed flow process. The flow of gas on the sheath side and a regenerating media through the bore may result in a pressure drop and/or cause bypass or channeling rendering the fibers useless. In addition, the complex post-treatment step can result in potential blockages at higher concentrations which may limit the barrier layer. The method may be useful in reducing the concentrations of CO2 from a high level of around 20% to about 1%, but it is not particularly useful for reducing the concentration to a very low level.
WO 2009/3174 describes a hollow fiber that may have a 50 micron or less thick barrier layer. The barrier layer may comprise “polyvinylidene chloride (PVDC), polyacrylonitrile, epichlorohydrin (Hydrin), polyether amide block co-polymer, glass, silica, alumina, metal, metal oxides, latex, other high barrier polymers, co-polymers thereof, or combinations thereof.” Unfortunately, some of these materials may prove difficult to employ and/or may be ineffective or inefficient in certain processes, for example, removing low level contaminants from a gas stream. They also may prove difficult to use for a suitable exterior barrier layer in some applications due to, for example, poor structural integrity as in the case of glass.
Thus, there is still a need for a material and process which is capable of removing low level contaminants from a gas stream both effectively and efficiently. It would further be beneficial if the material could be made in an efficient manner that does not require complex post-treatment steps.
Advantageously, the instant invention pertains to a material and process which is capable of removing low level contaminants from a gas stream both effectively and efficiently. Fortunately, the material may be made in a manner that does not require the intricacies of forming an interior barrier layer lining a lumen and its associated problems of potential blockages and capping requirements. And the material may be made efficiently and effectively in a multi-layer spinning process.
In one embodiment, the invention pertains to a fiber comprising a porous core and a sheath surrounding said porous core. The core comprises a sorbent and a polymer. The sheath is characterized by a heat resistance of at least about 110° C. and one or more of the following:
(1) a water vapor transmission rate (WVTR) of less than about 50, preferably less than about 25, preferably less than about 10, preferably less than about 3 Barrer at 38° C. and 90% relative humidity; or
(2) an O2 permeability of less than about 10, preferably less than about 3, preferably less than about 0.5, preferably less than about 0.02 Barrer; or
(3) an N2 permeability of less than about 1, preferably less than about 0.7, preferably less than about 0.005 Barrer; or
(4) a CO2 permeability of less than about 20, preferably less than about 5, preferably less than about 0.5, preferably less than about 0.1 Barrer.
In another embodiment, the invention pertains to a composition suitable for making a sheath on a fiber in an improved multi-layer spinning process. That is, in a multi-layer spinning process for making a sheath on a fiber comprising spinning a sheath dope with a core dope to form a fiber comprising a porous core and a sheath surrounding said porous core the improvement comprises employing a sheath dope composition which comprises a polyvinylidene chloride and a solvent comprising a dipolar aprotic solvent wherein the polyvinylidene chloride comprises at least about 15 weight percent based on the total weight of the composition. This facilitates a solution of the problem in the prior art wherein dilute post-treatment solutions had to be employed to avoid clumping of fibers.
In another embodiment, the instant invention pertains to a process for reducing the amount of one or more low level contaminants of a gas stream. The process comprises contacting the gas stream comprising an initial concentration of one or more low level contaminants with one or more fibers. One or more of the fibers comprises a porous core and a sheath surrounding said porous core. The core comprises a sorbent and a polymer. The sheath is characterized by a heat resistance of at least about 110° C. and one or more of the following:
(1) a water vapor transmission rate (WVTR) of less than about 50, preferably less than about 25, preferably less than about 10, preferably less than about 3 Barrer at 38° C. and 90% relative humidity; or
(2) an O2 permeability of less than about 10, preferably less than about 3, preferably less than about 0.5, preferably less than about 0.02 Barrer; or
(3) an N2 permeability of less than about 1, preferably less than about 0.7, preferably less than about 0.005 Barrer; or
(4) a CO2 permeability of less than about 20, preferably less than about 5, preferably less than about 0.5, preferably less than about 0.1 Barrer.
The contacting is conducted in a manner such that the initial concentration of one or more low level contaminants in the gas stream is reduced by sorption of said one or more low level contaminants on said sorbent. Next, one or more fibers is regenerated by passing a regenerating fluid which has a temperature of at least about 50° C. over the sheath layer of said fiber in a manner such that the fluid does not substantially contact the porous core.